Catalytic system for the reducton of nitrogen oxides

ABSTRACT

There is presented a process for the treatment of exhaust gas, which process uses a specially prepared catalyst composition, for the selective catalytic reduction of NO x  contained in the exhaust gas. An embodiment of the process of this invention comprises a catalytic stage to selectively catalytically reduce NO x  over a catalyst composition comprising a molecular sieve that has been treated with a metal in a way effective to maximize metal dispersion. The catalyst of this invention typically comprises a silica, titania, or zirconia binder, e.g. a binder including a high molecular weight, hydroxyl functional silicone resin. The catalyst of this invention may be formed into a desired shape, e.g., by extrusion, and finished in a humidified atmosphere after forming.

FIELD OF THE INVENTION

This invention is concerned with the abatement of nitrogen oxides and,optionally, other undesirable compounds, in industrial and engineexhaust gases. In particular, it is concerned with a catalytic methodfor efficiently eliminating these undesirable compounds before dischargeto the atmosphere. It is more particularly concerned with the use of aspecially prepared catalyst comprising a molecular sieve that has beenprepared from a physical mixture of the molecular sieve and the metalfor the selective catalytic reduction of the NO_(x) present in theexhaust gas.

BACKGROUND OF THE INVENTION

Although several nitrogen oxides are known which are relatively stableat ambient conditions, it is generally recognized that two of these,viz. nitric oxide (NO) and nitrogen dioxide (NO₂), are the principalcontributors to smog and other undesirable environmental effects whenthey are discharged into the atmosphere. These effects will not bediscussed further here since they are well recognized and have ledvarious governmental authorities to restrict industrial emissions in anattempt to limit the level of the oxides in the atmosphere. Nitric oxideand nitrogen dioxide, under appropriate conditions, are interconvertibleaccording to the equation.

    2NO+O.sub.2 ⃡2NO.sub.2

For purposes of the present invention, NO_(x) will be used herein torepresent nitric oxide, nitrogen dioxide, and nitrous oxide, as well asmixtures containing these gases.

Formation of man-made nitrogen oxides from the elements occurs in thehigh temperature zones of combustion processes. The internal combustionengine, and coal or gas-fired or oil-fired furnaces, boilers andincinerators, all contribute to NO_(x) emissions. In general, fuel-richcombustion mixtures produce exhaust gases with lower contents of NO_(x)than do lean mixtures. Although the concentrations of NO_(x) in theexhaust gases produced by combustion usually are low, the aggregateamount discharged in industrial and/or highly populated areas tends tocause problems. Other industrial sources of NO_(x) also exist. These areassociated with the manufacture of nitric acid, with nitration oforganic chemicals, and with other chemical operations such as thereprocessing of spent nuclear fuel rods by dissolution in nitric acid torecover uranyl nitrate followed by calcination to convert the nitrate touranium oxide. In these instances the waste gases may contain relativelyhigh levels of NO_(x), approaching 3%.

The so-called "stable" nitrogen oxides have in common the somewhatpeculiar property that although they are thermodynamically unstable withrespect to decomposition into elemental oxygen and nitrogen, no simple,economical method has been described for inducing this decomposition. Ithas been discovered, however, that the addition of a reductant such asammonia to the exhaust gas, under appropriate reaction conditions,converts NO_(x) to elemental nitrogen and steam and denitrifies theexhaust gas.

The process of contacting an industrial flue gas with a catalyst in thepresence of ammonia at a temperature in the range of about 200°-600° C.to denitrify the flue gas has come to be known as the process forSelective Catalytic Reduction (SCR) of NO_(x). In order to avoidconfusion, any reference made herein to "Selective Catalytic Reduction,"or to "SCR," is intended to refer to a process in which a mixture ofNO_(x) and NH₃ are induced to react catalytically at elevatedtemperatures. The term "denitrify" as used herein, means to reduce theamount of one or more noxious nitrogen compounds (such as NO, NO_(x) andHCN) contained in a waste gas, preferably by conversion to nitrogen gas.

The use of zeolite-based catalysts for the SCR of nitrogen oxides withammonia is well established. For example, U.S. Pat. No. 4,220,632 toPence et al. discloses a process for reducing noxious nitrogen oxidesfrom a fossil-fuel-fired power generation plant, or from otherindustrial plant off-gas streams, to elemental nitrogen and/or innocuousnitrogen oxides employing ammonia as the reductant and, as the catalyst,the hydrogen or sodium form of a zeolite having pore openings of about 3to 10 Angstroms.

U.S. Pat. No. 5,173,278 to Marler et al. discloses an SCR process wherethe ammonia needed for the reduction of NO_(x) is generated, at least inpart, by hydrolysis of HCN over a supported transition metal and/or acrystalline zeolite catalyst. The process described in this patentappears to require that HCN be present.

In particular, it is known that the hydrogen form of ZSM-5 (HZSM-5) iswell suited for this reaction at temperatures between about 400°-500° C.U.S. Pat. No. 4,778,665 to Krishnamurthy et al. describes an SCR processfor pre-treating industrial exhaust gases contaminated with NO_(x) inwhich the catalyst has a silica to alumina ratio of at least about 20and a Constraint Index of 1 to 12. The entire contents of this patentare incorporated herein by reference as if fully set forth.

At temperatures below about 400° C., HZSM-5 is significantly lessefficient at removing nitrogen oxides from the gas stream.

BRIEF SUMMARY OF THE INVENTION

One embodiment of this invention is a method for treating an exhaust gascomprising NO_(x) and ammonia, said method comprising directing theexhaust gas along with a source of oxygen over a catalyst under treatingconditions effective for the selective catalytic reduction of NO_(x) ;said catalyst comprising a molecular sieve which has been physicallymixed with a metal and a binder or binder precursor under contactingconditions effective to produce a metal loading with reference to themolecular sieve of about 0.01 wt. % to about 5 wt. %; said binder orbinder precursor comprising at least one selected from the groupconsisting of titania, zirconia, and silica; said catalyst having beenfinished in a humidified atmosphere.

Another embodiment of this invention is a method for treating an exhaustgas comprising NO_(x) and ammonia, said method comprising directing theexhaust gas along with a source of oxygen over a catalyst compositionunder treating conditions effective for the selective catalyticreduction of NO_(x) ; said catalyst composition having been prepared byphysically admixing into one formable mass:

(a) a molecular sieve;

(b) a metal oxide;

(c) a silicone resin;

(d) a methyl cellulose; and

(e) at least one carrier selected from the group consisting of methanol,ethanol, isopropyl alcohol, N-methyl pyrrolidone, dibasic ester, waterand mixtures thereof;

said catalyst composition further having been formed into a desiredshape, said catalyst composition still further having been finished in ahumidified atmosphere after being formed.

Yet another embodiment of this invention is a method of making acatalyst composition suitable for the selective catalytic reduction ofNO_(x) comprising: producing a formable mass by physically admixing

(a) a molecular sieve;

(b) a metal;

(c) a silicone resin;

(d) a methyl cellulose; and

(e) at least one suitable carrier selected from the group consisting ofmethanol, ethanol, isopropyl alcohol, N-methyl pyrrolidone, dibasicester, water and mixtures thereof;

extruding the formable mass into a desired shape, and then finishing theextruded shape in a humidified atmosphere.

DETAILED DESCRIPTION

The term "exhaust gas" as used herein means any waste gas which isformed in an industrial process or operation and which is normallydisposed of by discharge to the atmosphere, with or without additionaltreatment. "Exhaust gas" also includes the gas produced by internalcombustion engines. The composition of such a gas varies and depends onthe particular process or operation which leads to its formation. Whenformed in the combustion of fossil fuels, it will generally comprisenitrogen, steam and carbon dioxide in addition to low levels, such as upto about 1000 ppm, of nitric oxide plus nitrogen dioxide.Sulfur-containing fuels will typically produce an exhaust gas thatcontains one or more sulfur oxides, such as SO₂. Rich fuel-air mixtureswill generally produce an exhaust gas that contains little if any freeoxygen along with some carbon monoxide, hydrocarbons, and hydrogen. Leanfuel-air mixtures, i.e., mixtures in which more air is provided than isstoichiometrically required to completely burn the fuel, will form anexhaust gas that contains gaseous oxygen. The foregoing is a generaldescription given to illustrate the variability in the composition ofthe exhaust gases from fossil fuel combustion. Other industrialprocesses such as nitration, uranium recovery, and calcining nitratesalt containing solids produce exhaust gases which can have compositionsdifferent from those noted above. They may be substantially devoid ofsteam, for example, and may contain very high concentrations of nitrogenor other inert gases.

The conversion of NO_(x) to N₂ is believed to proceed generallyaccording to equations (1) and (2).

    2NO.sub.2 +4NH.sub.3 +O.sub.2 →3N.sub.2 +6H.sub.2 O (1)

    4NO+4NH.sub.3 +O.sub.2 →4N.sub.2 +6H.sub.2 O        (2)

This invention is effective for treating exhaust gas containing theapproximate stoichiometric amount of ammonia. The ammonia may be presentin the gas, may be added to the gas, or may be produced by an upstreamprocess. As used herein, the expression "approximate stoichiometricamount of ammonia" is intended to mean about 0.75 to about 1.25 timesthe molar amount of ammonia indicated in equations (1) and (2) whenexcess oxygen is present.

The catalyst of this invention provides significantly improved SCRactivity at relatively low temperatures, e.g., below about 400° C.Additionally, little or no ammonia oxidation is observed at highertemperatures, e.g., above about 500° C., such as has been seen withother metal containing catalysts when used for the SCR of NO_(x).

According to the method of this invention, any carbon monoxide andhydrocarbons present in the exhaust gas may be oxidized to carbondioxide and water over the catalyst. Additionally, hydrocarbons may beselectively absorbed/adsorbed on the catalyst.

One embodiment of the invention is a method for treating a gas mixturecomprising NO_(x), ammonia, and, optionally, at least one of CO and ahydrocarbon and mixtures thereof, said method comprising directing thegas mixture along with a source of oxygen over a catalyst underconditions effective for the selective catalytic reduction of NO_(x),said catalyst comprising a molecular sieve, which has had a metal addedunder conditions effective to provide maximum metal dispersion, e.g., byphysically contacting the molecular sieve with a metal, said catalysthaving been finished in a humidified atmosphere, and said catalystoptionally further comprising a binder.

In another embodiment of the invention, a water insoluble metal oxide,e.g., iron oxide (Fe₂ O₃), and a molecular sieve, e.g., ZSM-5, arephysically mixed in the presence of a binder precursor, such as asilicone resin, and the mixture is formed into a desired shape, such asby extrusion, and the formed shape is dried and then finished in ahumidified atmosphere.

Each of the principal features of this invention will be more fullydescribed below.

Feeds

This invention is effective to treat industrial and engine exhaust gasesto remove NO_(x), and optionally other undesirable compounds, such as COand hydrocarbons, if present. These exhaust gases are typically producedin internal combustion engines, and coal or gas-fired or oil-firedfurnaces, boilers and incinerators, and by the manufacture of nitricacid, by the nitration of organic chemicals, and by other chemicaloperations such as the reprocessing of spent nuclear fuel rods bydissolution in nitric acid to recover uranyl nitrate followed bycalcination to convert the nitrate to uranium oxide.

Process Conditions

The exhaust gas is typically treated in the catalytic system of thisinvention at a temperature of about 200° C. to about 1,000° C. or more,e.g. within the range of about 225° C. to about 900° C., e.g. of about225° C. to about 750° C., e.g. of about 250° C. to about 600° C. and ata gas hourly space velocity, GHSV, (vols. of gas at STP per volume ofcatalyst per hour) adjusted to provide the desired conversion. The GHSVcan be from about 1,000 to about 500,000 hr⁻¹, e.g. within the range ofabout 2,500 to about 250,000 hr⁻¹, e.g. of from about 5,000 to about150,000 hr⁻¹, e.g. of from about 10,000 to about 100,000 hr⁻¹. Theprocess of this invention is operable at subatmospheric tosuperatmospheric pressure, e.g. at about 5 to about 500 psia, e.g. atabout 10 to about 50 psia, i.e. near or slightly above atmosphericpressure.

The gas mixture directed over the catalyst should contain at least astoichiometric amount of oxygen as indicated by equations (1) and (2)above. Excess levels of oxygen above the stoichiometric amount aredesirable. If sufficient oxygen is not present in the exhaust gas, asource of oxygen, e.g. air, may be added to the exhaust gas, and ifsufficient oxygen is present in the exhaust gas, no air need be added tothe exhaust gas.

Adequate conversion may be readily achieved with a simple stationaryfixed-bed of catalyst. However, other contacting means are alsocontemplated, such as contacting with a fluid bed, a transport bed, anda monolithic catalyst structure such as a honeycomb.

Suitable mixing may be used before the catalytic stage of this inventionto produce a homogeneous gas mixture for reaction in that stage. Themixers may be any suitable arrangement, including, for example, baffles,discs, ceramic discs, static mixers or combinations of these.

Catalyst Composition

Catalysts useful in this invention typically comprise an active materialand a support or binder. The support for the catalysts of this inventionmay be the same as the active material and further can be a synthetic ornaturally occurring substance as well as an inorganic material such asclay, silica and/or one or more metal oxides. The latter may be eithernaturally occurring or in the form of gelatinous precipitates or gelsincluding mixtures of silica and metal oxides. Naturally occurring clayswhich can be used as support for the catalysts include those of themontmorillonite and kaolin families, which families include thesubbentonites and the kaolins commonly known as Dixie, McNamee, Georgiaand Florida clays or others in which the main mineral constituent ishalloysite, kaolinite, dickite, nacrite or anauxite. Such clays can beused in the raw state as originally mined or initially subjected tocalcination, acid treatment or chemical modification. In addition to theforegoing materials, the catalysts of this invention may be supported ona porous binder or matrix material, such as titania, zirconia,silica-magnesia, silica-zirconia, silica-thoria, silica-berylia,silica-titania, titania-zirconia, as well as a ternary compound such assilica-magnesia-zirconia. A mixture of these components could also beused. The support may be in the form of a cogel. One binder that issuitable is a low acidity titania prepared from a mixture comprising alow acidity titanium oxide binder material and an aqueous slurry oftitanium oxide hydrate. Other binders include alumina andalumina-containing materials such as silica-alumina,silica-alumina-thoria, silica-alumina-zirconia, andsilica-alumina-magnesia. Typical aluminas include alpha (α) alumina,beta (β) alumina, gamma (γ) alumina, chi-eta-rho (χ,η,ρ) alumina, delta(δ) alumina, theta (θ) alumina, and lanthanum beta (β) alumina. Thepreferred support is one that is a high surface area material that alsopossesses a high temperature stability and further possesses a highoxidation stability.

The binder may be prepared according to application U.S. Ser. No.08/112,501, now U.S. Pat. No. 5,430,000 incorporated by referenceherein, or may be prepared according to methods disclosed in U.S. Pat.Nos. 4,631,267; 4,631,268; 4,637,995; and 4,657,880, each incorporatedby reference herein. Also, the catalysts described herein may becombined with any of the binder precursors described in the aboveapplication and patents, and then may be formed, such as by extrusion,into the shape desired, and then finished in a humidified atmosphere ashereinafter described. The preferred binder is substantially free ofalumina. By the term "substantially free of alumina" is meant that noalumina is intentionally added to the binder, however, it is recognizedthat trace amounts of alumina may be present.

When low acidity titania is used as a binder, it is desirable that theformable, e.g., extrudable, mass prepared by combining the zeolite, theiron salt, and the titania binder precursors contain at least about 0.5wt. %, typically from about 1 wt. % to about 20 wt. %, e.g., from about2 to about 8 wt. % of the aqueous slurry of titanium oxide hydrate.

The low acidity titania is typically added in dry particulate form,e.g., titanium oxide hydrate, so as to control the moisture content ofthe binder/dispersant mixture at a level to promote satisfactoryforming, e.g., extrusion.

The catalysts may also contain stabilizers such as alkaline earthoxides, phosphates and combinations thereof.

Catalysts of this invention are frequently used with a substrate. Amaterial can be both substrate and part of the catalyst. Suitablesubstrate materials include cordierite, nitrides, carbides, borides,intermetallics, mullite, alumina, natural and synthetic zeolites,lithium aluminosilicate, titania, feldspars, quartz, fused or amorphoussilica, clays, aluminates, zirconia, spinels, or metal monoliths ofaluminum-containing ferrite type stainless steel, or austenite typestainless steel, and combinations thereof. Typical substrates aredisclosed in U.S. Pat. Nos. 4,127,691 and 3,885,977, incorporated byreference herein. The catalyst may be combined with the substrate in anymethod that ensures that the catalyst will remain intact during thecatalytic reaction. For example, the catalyst may be present as acoating on the substrate, or it can be present as an integral part ofthe substrate. Additionally, as mentioned earlier, the substrate and atleast part of the catalyst may be the same. For example, in someembodiments, zeolites may be used as both catalysts and substrates. Whenthe catalyst of this invention is deposited on the substrate, it may bedone using a wash coat. The wash coat may be prepared, for example, byadding silica sol and water to the catalyst powder, mulling the mixtureto form a thixotropic slurry, dipping the monolithic substrate into theslurry, and then drying and calcining the resulting structure.Alternatively, the catalyst may be formed and extruded together with thesubstrate and thus may become an integral part of the substrate.

The form and the particle size of the catalyst is not critical to thepresent invention and may vary depending, for example, on the type ofreaction system employed. Non-limiting examples of the shapes of thecatalyst for use in the present invention include balls, pebbles,spheres, extrudates, channeled monoliths, honeycombed monoliths,microspheres, pellets or structural shapes, such as lobes, pills, cakes,honeycombs, powders, granules, and the like, formed using conventionalmethods, such as extrusion or spray drying. Where, for example, thefinal particles are designed for use as a fixed bed, the particles maybe formed into particles having a minimum dimension of at least about0.01 inch and a maximum dimension of up to about one-half inch or oneinch or more. Spherical particles having a diameter of about 0.03 inchto about 0.25 inch, e.g., about 0.03 inch to about 0.15 inch, are oftenuseful, especially in fixed bed or moving bed operations. With regard tofluidized bed systems, the major amount by weight of the particles mayhave a diameter in the range of about 10 microns to about 250 microns,e.g., about 20 microns to about 150 microns.

The gas mixture, as described above, is contacted with a catalystcomprising a molecular sieve catalyst having the properties describedbelow. The molecular sieve useful in this invention is not limited toany particular molecular sieve material and, in general, includes allmetallosilicates, metallophosphates, silicoaluminophosphates, andlayered and pillared layered materials, which effectively catalyze theselective catalytic reduction reaction of the present invention.Particularly useful are the aluminosilicates whether or not previouslydealuminized to increase the framework silica:alumina ratio. Typicalzeolites include ZSM-4 (Omega), ZSM-5, ZSM-11, ZSM-12, ZSM-20, ZSM-22,ZSM-23, ZSM-35, ZSM-48, ZSM-50, ZSM-57, ZSM-58, MCM-22, PSH-3, Beta, X,Y, and L, as well as ferrierite, mordenite, dachiardite, clinoptilolite,offretite, erionite, gmelinite, chabazite, etc. Other molecular sievescontemplated include, for example, MCM-9, VPI-5, MCM-20, SAPO-11,SAPO-17, SAPO-34, SAPO-37, and MCM-41. Particularly useful are theintermediate pore size zeolites, e.g., those less than about 7 Angstromspore size, such as from about 5 to less than about 7 Angstroms, e.g.,those which have a Constraint Index between about 1 and about 12.

It is to be understood that the identification of the molecular sieves,e.g., zeolites, may be resolved on the basis of their respective X-raydiffraction patterns. The present invention contemplates utilization ofsuch molecular sieves wherein the mole ratio of silica-to-metal oxide isessentially unbounded. The molecular sieves are not limited to specificsilica:metal oxide mole ratios and, yet, having the same crystalstructure as the disclosed materials, may be useful or even preferred insome applications. It is the crystal structure, as identified by theX-ray diffraction "fingerprint," which establishes the identity of thespecific molecular sieve, e.g., zeolite, material.

Examples of intermediate pore size zeolites include ZSM-5 (U.S. Pat. No.3,702,886 and Re. 29,948); ZSM-11 (U.S. Pat. No. 3,709,979); ZSM-12(U.S. Pat. No. 3,832,449); ZSM-21 (U.S. Pat. No. 4,046,859); ZSM-22(U.S. Pat. No. 4,556,477); ZSM-23 (U.S. Pat. No. 4,076,842); ZSM-35(U.S. Pat. No. 4,016,245); ZSM-38 (U.S. Pat. No. 4,406,859); ZSM-48(U.S. Pat. No. 4,397,827); ZSM-57 (U.S. Pat. No. 4,046,685); and ZSM-58(U.S. Pat. No. 4,417,780). The entire contents of the above referencesare incorporated by reference herein.

A characteristic of the crystal structure of this class of zeolites isthat it provides constrained access to and egress from theintracrystalline free space by virtue of having an effective pore sizeintermediate between the small pore Linde A and the large pore Linde X,i.e. the pore windows of the structure typically have a size such aswould be provided by 10-membered rings of oxygen atoms. It is to beunderstood, of course, that these rings are those formed by the regulardisposition of the tetrahedra making up the anionic framework of themolecular sieve, the oxygen atoms themselves being bonded to the siliconor aluminum atoms at the centers of the tetrahedra.

The intermediate pore size zeolites referred to herein have an effectivepore size such as to freely sorb normal hexane. In addition, thestructure must provide constrained access to larger molecules. It issometimes possible to judge from a known crystal structure whether suchconstrained access exists. For example, if the only pore windows in acrystal are formed by 8-membered rings of oxygen atoms, then access tomolecules of larger cross-section than normal hexane is excluded and thezeolite is not an intermediate pore size material. Windows of10-membered rings are preferred, although in some instances excessivepuckering of the rings or pore blockage may render these zeolitesineffective.

Rather than attempt to judge from crystal structure whether or not azeolite possesses the necessary constrained access to molecules largerthan normal paraffins, a simple determination of the Constraint Indexmay be made. The method by which Constraint Index is determined isdescribed fully in U.S. Pat. No. 4,016,218, incorporated herein byreference for details of the method. Constraint Index (CI) values forsome typical zeolites including some which are suitable as catalysts inthe process of this invention are as follows:

    ______________________________________                                        CI (at test temperature)                                                      ______________________________________                                        ZSM-4             0.5      (316° C.)                                   ZSM-5             6-8.3    (371° C.-316° C.)                    ZSM-11            5-8.7    (371° C.-316° C.)                    ZSM-12            2.3      (316° C.)                                   ZSM-20            0.5      (371° C.)                                   ZSM-22            7.3      (427° C.)                                   ZSM-23            9.1      (427° C.)                                   ZSM-34            50       (371° C.)                                   ZSM-35            4.5      (454° C.)                                   ZSM-48            3.5      (538° C.)                                   ZSM-50            2.1      (427° C.)                                   MCM-22            0.6-1.5  (399° C.-454° C.)                    TMA Offretite     3.7      (316° C.)                                   TEA Mordenite     0.4      (316° C.)                                   Clinoptilolite    3.4      (510° C.)                                   Mordenite         0.5      (316° C.)                                   REY               0.4      (316° C.)                                   Amorphous Silica-alumina                                                                        0.6      (538° C.)                                   Dealuminized Y    0.5      (510° C.)                                   Erionite          38       (316° C.)                                   Zeolite Beta      0.6-2.0  (316° C.-399° C.)                    ______________________________________                                    

The above-described Constraint Index is one definition of thoseintermediate pore size zeolites which are useful in the process of thepresent invention. The very nature of this parameter and theabove-referenced procedure by which it is determined, however, admits ofthe possibility that a given zeolite can be tested under somewhatdifferent conditions and thereby exhibit different Constraint Indices.Constraint Index appears to vary somewhat with the severity of theconversion operation and the presence or absence of support material.Similarly, other variables such as crystal size of the zeolite, thepresence of occluded contaminants, etc., may affect the observedConstraint Index value. It will therefore be appreciated that it may bepossible to select test conditions, e.g., temperature, as to establishmore than one value for the Constraint Index of a particular zeolite.This explains the range of Constraint Indices for some zeolites, such asZSM-5, ZSM-11, MCM-22, and Beta.

The catalyst may include a support and may be used with a substrate. Inaccordance with one embodiment, the catalyst may be brought into contactwith a monolithic ceramic substrate by crystallizing the zeolite on thesurface of the substrate, as disclosed in U.S. Pat. No. 4,800,187,incorporated herein by reference. The catalyst may also be formed intothe substrate, such as by extrusion.

A catalyst that is useful in the process of this invention is preparedby combining into a single mixture a zeolite, such as the hydrogen formof ZSM-5, a metal oxide, such as iron oxide (Fe₂ O₃), a high molecularweight, hydroxy functional silicone, such as Dow Corning Q6-2230silicone resin, a suitable extrusion aid, such as methyl cellulose, anda suitable polar, water soluble carrier, such as methanol, ethanol,isopropyl alcohol, N-methyl pyrrolidone or a dibasic ester along withwater as needed, then forming the mixture into the desired shape, suchas by extrusion, then finishing the formed material by treating it in ahumidified atmosphere. One particular methyl cellulose that is effectiveas an extrusion aid in the method of this invention is a hydroxypropylmethyl cellulose, such as K75M Methocel™, available from Dow ChemicalCo. Dibasic esters that are useful in this invention include dimethylglutarate, dimethyl succinate, dimethyl adipate, and mixtures thereof,one example of which is E. I. DuPont de Nemours & Co. DBE, whichtypically comprises about 50 to 75 percent dimethyl glutarate, 10 to 25percent dimethyl adipate, 19 to 26 percent dimethyl succinate and lessthan about 0.2 wt. % methanol. Other silicone resins that may be used inthe method of this invention include those described in U.S. Pat. No.3,090,691.

The relative proportions of molecular sieve component and the supportmaterial on an anhydrous basis may vary widely with the zeolite contentranging from between about 5 to about 99 percent by weight and moreusually in the range of about 10 to about 95 percent by weight, e.g.,from about 20 to about 90 percent by weight of the dry composite.

Original ions, e.g., alkali or alkaline earth metal, of theas-synthesized molecular sieve and any found in the molecularsieve/support material can be replaced in accordance with techniqueswell known in the art, at least in part, by ion-exchange with otherions. For the present catalyst composition, potential replacing ionsinclude hydrogen ions and hydrogen precursor, e.g., ammonium ions. ZSM-5in the hydrogen exchanged form is referred to herein as HZSM-5. Typicalion-exchange techniques would be to contact the molecular sieve ormolecular sieve/support material with a solution containing a salt ofthe desired replacing ion or ions. Examples of such salts include thehalides, e.g., chlorides, nitrates and sulfates. Representativeion-exchange techniques are disclosed in a wide variety of patentsincluding U.S. Pat. Nos. 3,140,249; 3,140,251; and 3,140,253,incorporated by reference herein.

The desired metal loading on the molecular sieve component of thecatalyst is that amount effective to yield about 0.01 to about 5 wt. %,e.g. within this range of at least about 0.4 wt. %, e.g., at least about0.6 wt. %, e.g., at least about 1 wt. %, e.g., at least about 1.5 wt. %,e.g., about 2 wt. %, elemental metal based upon the molecular sieve.Potential metals include one or more of those metals, such as transitionmetals, possibly a noble metal, which are able to oxidize otherundesirable compounds present in the exhaust gas along with promotingthe SCR of NO_(x). The metal is typically selected from at least one ofthe metals of Groups of the Periodic Table IIIA, IB, IIB, VA, VIA, VIIA,VIIIA, and combinations thereof. Examples of these metals include atleast one of copper, zinc, vanadium, chromium, manganese, cobalt, iron,nickel, rhodium, palladium, platinum, molybdenum, tungsten, cerium andmixtures thereof. An example of a subset of these metals is one or moreof the noble metals, platinum, palladium and combinations of these,along with iron and cerium. The above described metals may be usedindividually and in combination with each other. The term "metal" asused herein is intended to include the elemental metal as well as metaloxides, metal sulfides, and other metal containing compounds.

Metal oxides useful in this invention include at least one of the copperoxides, including copper peroxide (CU₂ O₃), cupric oxide (CuO), andcuprous oxide (Cu₂ O); zinc oxide (ZnO); the vanadium oxides, includingvanadium oxide (VO), vanadium dioxide (VO₂), vanadium trioxide (VO₃),vanadium tetroxide (VO₄), vanadium pentoxide (V₂ O₅); the chromiumoxides, including chromium dioxide (CrO₂), chromium trioxide (CrO₃),chromic oxide (Cr₂ O₃), chromous oxide (CrO); the manganese oxides,including manganous oxide (MnO), manganic manganous oxide (Mn₃ O₄),manganese trioxide (MnO₃), manganese dioxide (MnO₂), manganese heptoxide(Mn₂ O₇); the cobalt oxides, including cobaltous oxide (CoO), cobalticoxide (Co₂ O₃), cobalto cobaltic oxide (Co₃ O₄); the iron oxides,including ferrous oxide (FeO), ferric oxide (Fe₂ O₃), ferriferrous oxide(Fe₃ O₄); the nickel oxides, including nickelous oxide (NiO), nickelicoxide (Ni₂ O₃), nickelous nickelic oxide (Ni₃ O₄), nickel peroxide(NiO₂), nickel super oxide (NiO₄); the palladium oxides, includingpalladium monoxide (PdO), palladium dioxide (PdO₂); the platinum oxides,including platinous oxide (PtO), platinum dioxide (PtO₂); the molybdenumoxides, including molybdenum dioxide (MoO₂), molybdenum sesquioxide (Mo₂O₃), molybdenum trioxide (MoO₃); the tungsten oxides, including tungstendioxide (WO₂), tungsten trioxide (WO₃), tungsten pentoxide (W₂ O₅), thecerium oxides, including cerium dioxide (CeO₂), cerous oxide (Ce₂ O₃),and combinations thereof. The water insoluble metal oxides areappropriate for use in the method of this invention.

If desired, alkali metals or alkaline-earth metals, including sodium,potassium, rubidium, cesium, magnesium, calcium, and barium, may also bepresent in the catalyst or may be added to the catalyst.

After the molecular sieve has been treated with the metal and, possibly,formed into any desired shape, the metal treated molecular sieve may befinished by treatment in a humidified atmosphere. This "finish"treatment may include calcination or thermal treatment in air, or in aninert gas, at temperatures ranging from about 260° C. to about 925° C.for periods of time ranging from about 1 to about 48 hours or more,e.g., at about 538° C. for about 4 to about 6 hours. Whilesubatmospheric or superatmospheric pressure can be employed for thethermal treatment, atmospheric pressure is useful simply for reasons ofconvenience.

Also included in this "finish" treatment is treating the catalyst withgas streams containing steam. Catalysts of improved selectivity andother beneficial properties, such as improved hydrothermal stability,can be obtained by subjecting the metal treated molecular sieve to atleast one treatment with streams containing steam (hydrothermallytreating the catalysts) at elevated temperatures ranging from about 260°C. to about 900° C., specifically from about 400° C. to about 850° C.,more specifically from about 500° C. to about 700° C. The hydrothermaltreatment may be accomplished in an atmosphere containing at least 20ppm, 0 5%, 5%, 10%, 20% and even up to about 99% steam in air or someother suitable gas stream, such as nitrogen or some other gas which isessentially inert to the zeolite. Optionally, more than one hydrothermaltreatment may be used, e.g., two, three, or more hydrothermal treatmentsat different temperatures, e.g., increasing temperatures, may be used.Typical steaming conditions are described in U.S. Pat. Nos. 4,429,176;4,522,929; 4,594,146; and 4,663,492; each incorporated by referenceherein. The calcination and hydrothermal treatments of the catalysts maybe combined into one treatment step and conducted simultaneously.

The combination of the metal and the molecular sieve may be accomplishedby contacting the molecular sieve with a metal, such as those mentionedabove. In this method, the metal and the molecular sieve, and any binderor binder precursor desired, may be physically combined to produce amixture and the mixture recovered and formed, such as by extrusion. Ifdesired, water or another suitable carrier or solvent may also be addedto the mixture. The formed material may be dried, and then finished in ahumidified atmosphere as is more fully described herein.

As noted above, the catalytic reduction of nitrogen oxides issubstantially effected by the use of the present process. Bysubstantially effected is meant a conversion of greater than about 40,80, 85, 90, 95, or even 99% or more of the nitrogen oxides and theammonia in the exhaust gas to innocuous compounds, such as nitrogen,through the use of this process. This is also referred to herein asconversion of a substantial portion of the NO_(x) and ammonia in theexhaust gas to innocuous compounds.

The catalysts of this invention will now be illustrated by examples. Theexamples are for illustrative purposes only and are not to be construedas limiting the scope of the invention, which scope is defined by thisentire specification including the appended claims.

An unmodified, untreated hydrogen form ZSM-5 catalyst was used as thebase catalyst for all of the following examples and was also used as thereference catalyst for comparison examples, where appropriate.

EXAMPLE 1

A ZSM-5 catalyst was prepared by the following method: 99 grams ofcalcined ZSM-5 were mixed in a muller with 20 grams of Dow CorningQ6-2230 silicone resin, and 6.5 grams of Dow Chemical Co. K75MMethocel™. To this dry blend, 52.9 grams of distilled water and 23.1grams of E. I. DuPont de Nemours & Co. DBE (dibasic ester) were addedwhile mulling. The mixture was then extruded to form 1/16 inchcylindrical extrudates. The extrudates were dried overnight at 120° C.and then calcined at 600° C. in 10% steam for 10 hours to produce aZSM-5 containing catalyst. This catalyst is referred to herein asCatalyst A.

EXAMPLE 2

An iron containing ZSM-5 sample was prepared by the following method: 99grams of calcined ZSM-5 were mixed in a muller with 20 grams of DowCorning Q6-2230 silicone resin, 6.5 grams of Dow K75M Methocel™, and 3.2grams of iron oxide, Fe₂ O₃. To this dry blend, 52.9 grams of distilledwater and 23.1 grams of DuPont DBE (dibasic ester) were added whilemulling. The mixture was then formed into 1/16 inch cylindricalextrudates. The extrudates were dried overnight at 120° C. and thencalcined at 600° C. in 10% steam for 10 hours to produce an ironcontaining catalyst, Catalyst B.

EXAMPLE 3

In this example, the SCR activity of Catalyst A is compared with the SCRactivity of Catalyst B. The catalyst samples were evaluated using afixed-bed quartz reactor operating between 250° and 550° C. The reactorwas loaded with 2.75 grams of catalyst with inlet gases consisting of500 ppm NO, 500 ppm NH₃, and 5% O₂ in a N₂ carrier flowing at a constantflow rate of 1,000 cc/min. The effluent from the reactor wascontinuously monitored by FTIR (Fourier Transform Infrared) analysis.Catalyst activity results are summarized below in Table 1.

                  TABLE 1                                                         ______________________________________                                        Net NO Conversion, %                                                          Temperature, °C.                                                                       Catalyst A                                                                              Catalyst B                                          ______________________________________                                        550             71%       92%                                                 454             74%       96%                                                 398             72%       96%                                                 343             60%       96%                                                 250             25%       65%                                                 ______________________________________                                    

EXAMPLE 4

An iron containing ZSM-5 sample was prepared by the following method: 99grams of calcined ZSM-5 were mixed in a muller with 20 grams of DowCorning Q6-2230 silicone resin, 6.5 grams of Dow K75M Methocel™, and0.32 grams of iron oxide, Fe₂ O₃. To this dry blend, 52.9 grams ofdistilled water and 23.1 grams of DuPont DBE (dibasic ester) were addedwhile mulling. The mixture was then formed into 1/16 inch cylindricalextrudates. The extrudates were dried overnight at 120° C. and thencalcined at 600° C. in 10% steam for 10 hours to produce an ironcontaining catalyst, Catalyst C.

EXAMPLE 5

In this example, the SCR activity of Catalyst A is compared with the SCRactivity of Catalyst C. The catalyst samples were evaluated using afixed-bed quartz reactor operating between 250° and 550° C. The reactorwas loaded with 2.75 grams of catalyst with inlet gases consisting of500 ppm NO, 500 ppm NH₃, and 5% O₂ in a N₂ carrier flowing at a constantflow rate of 1,000 cc/min. The effluent from the reactor wascontinuously monitored by FTIR (Fourier Transform Infrared) analysis.Catalyst activity results are summarized below in Table 2.

                  TABLE 2                                                         ______________________________________                                        Net NO conversion, %                                                          Temperature, °C.                                                                       Catalyst A                                                                              Catalyst C                                          ______________________________________                                        550             71%       84%                                                 454             74%       86%                                                 398             72%       87%                                                 343             60%       82%                                                 250             25%       40%                                                 ______________________________________                                    

We claim:
 1. A method for treating an exhaust gas comprising NO_(x) andammonia, said method comprising directing the exhaust gas along with asource of oxygen over a catalyst under treating conditions effective forthe selective catalytic reduction of NO_(x) ; said catalyst comprising amolecular sieve which has been physically mixed with a metal and abinder or a binder precursor under contacting conditions effective toproduce a metal loading of about 0.01 wt. % to about 5 wt. % withreference to the molecular sieve; said molecular sieve being at leastone selected from the group consisting of ZSM-5, ZSM-11, ZSM-12, ZSM-21,ZSM-22, ZSM-23, ZSM-35, ZSM-38, ZSM-48, ZSM-58, and combinationsthereof; said binder or binder precursor comprising at least oneselected from the group consisting of titania, zirconia, and silica;said process further comprising finished said catalyst in a humidifiedatmosphere under conditions effective to produce improved hydrothermalstability; the metal being selected from the group consisting of atleast one of the Groups of the Periodic Table of IIIA, IB, IIB, VA, VIA,VIIA, VIIIA, and combinations thereof.
 2. The method according to claim1 wherein the molecular sieve comprises ZSM-5.
 3. The method accordingto claim 1 wherein the metal loading of the catalyst is at least about1.5 wt. % with reference to the molecular sieve.
 4. The method accordingto claim 3 wherein the metal loading of the catalyst is less than about3 wt. % with reference to the molecular sieve.
 5. The method accordingto claim 1 wherein the treating conditions comprise a temperature ofabout 200° C. to about 1,000° C., a pressure of about 5 to about 500psia, and a gas hourly space velocity (GHSV) of about 1,000 to about500,000 hr⁻¹.
 6. The method according to claim 1 wherein the source ofoxygen comprises air.
 7. The method according to claim 1 wherein themetal is selected from at least one of the oxides of metals of Groups ofthe Period Table IIIA, IB, IIB, VA, VIA, VIIA, VIIIA and combinationsthereof.
 8. The method according to claim 1 wherein the metal isselected from at least one of the water insoluble oxides of copper,zinc, vanadium, chromium, manganese, cobalt, iron, nickel, rhodium,palladium, platinum, molybdenum, tungsten, cerium and mixtures thereof.9. The method according to claim 1 wherein the metal comprises ironoxide.
 10. The method according to claim 1 wherein the binder issubstantially free of alumina, and wherein the molecular sieveconstitutes about 20 to about 90 weight percent of the finishedcatalyst.
 11. The method according to claim 1 wherein the humidifiedatmosphere comprises about 20 ppm to about 99% steam at about 260° C. toabout 900° C.